1. Field of the Invention
The present invention relates to a semiconductor device equipped with a primary semiconductor element and a temperature detecting element.
2. Description of the Related Art
It is desirable that a semiconductor device used for power switching is provided with an overheat protection function to prevent thermal breakdown of the semiconductor device due to an over-current. A function using change of forward and reverse characteristics of a diode in accordance with temperature is commonly known as the overheat protection function. For example, the forward voltage and the saturation voltage of a diode vary substantially linearly in accordance with temperature. Therefore, when a semiconductor device and a diode are provided as a main element (hereinafter referred to as primary semiconductor element) and a temperature detecting element respectively, the temperature of the primary semiconductor element can be detected by monitoring the forward voltage or the saturation voltage of the diode (e.g. as disclosed in U.S. Pat. No. 4,903,106, Japanese patent documents JP-A-2006-324412 and JP-A-2006-302977, and Japanese patent number 3,538,505). By reducing the gate voltage of the primary semiconductor element to limit the current flowing in the primary semiconductor element in response to the detection of the high temperature state of the primary semiconductor element, the primary semiconductor element can be protected from breakdown due to overheating.
FIG. 1 is a sectional view showing the configuration of a semiconductor device according to the related art. As shown in FIG. 1, the semiconductor device according to the related art has an N− drift layer 3, a surface structure of a primary semiconductor element 1, and a temperature detecting diode 2. The surface structure of the primary semiconductor element 1 and the temperature detecting diode 2 are provided in a first principal surface of the N− drift layer 3. The surface structure of the primary semiconductor element 1 has a P-base region 4a, an N+ emitter (source) region 5, a gate insulating film 6, a gate electrode 7, and an emitter (source) electrode 8. The temperature detecting element 2 has a P-type anode region (a P-base region 4b and a P+ region 9), an N+ cathode region 10, an anode electrode (not shown), and a cathode electrode (not shown).
Alternatively, a semiconductor device having a primary semiconductor element 1, an insulating film 11 formed in a first principal surface of a constituent semiconductor member of the primary semiconductor element 1, and a temperature detecting diode 2 formed on the insulating film 11 is commonly known as represented by a semiconductor device shown in FIG. 2 (e.g. as shown in Japanese patent document JP-A-6-117942). In addition, a power module in which a thermistor for detecting heat emitted from a switching circuit and a rectifier circuit is disposed nearby the switching circuit and the rectifier circuit is commonly known (e.g. as shown in Japanese patent document JP-A-2005-286270). In this specification and accompanying drawings, a layer or a region prefixed with N or P means that electrons or holes are majority carriers therein, respectively. Moreover, a layer or region with a superscript “+” or “−” attached to N or P indicates that the impurity concentration of the layer or region is higher or lower, respectively, than that of a layer or region without any superscript “+” or “−” attached to N or P.
In the semiconductor device shown in FIG. 1, a parasitic diode is however formed from the anode region of the temperature detecting diode and the N− drift layer. When a channel is formed in the primary semiconductor element, a current flowing in the channel also flows in the parasitic diode. For this reason, there is a problem that the forward voltage or the saturation voltage of the temperature detecting diode varies according to whether the primary semiconductor element is in an ON state or in an OFF state. In addition, when the primary semiconductor element is an IGBT (Insulated Gate Bipolar Transistor), a parasitic thyristor is formed from a P collector layer in a second principal surface, the N− drift layer and the P-type anode region and N+ cathode region of the temperature detecting diode. When this IGBT is turned OFF, there is a risk that latch-up breakdown will be caused by malfunction of the parasitic thyristor because holes which are minority carriers are imported from the N− drift layer into the anode region.
A configuration in which the primary semiconductor element and the temperature detecting diode are electrically insulated and separated has been disclosed in the aforementioned Japanese patent number 3,538,505. In this configuration, a parasitic thyristor is however formed from the primary semiconductor element and the temperature detecting diode. For this reason, there is a problem that the parasitic thyristor causes latch-up breakdown in the case where voltage change (dV/dt) at the time of switching is large and in the case where the current quantity is large.
In the semiconductor device shown in FIG. 2, the forward voltage (or the saturation voltage) of the temperature detecting diode varies because the temperature detecting diode is formed from polycrystalline silicon (polysilicon). In addition, the dependence of the ON-voltage on temperature deviates from a theoretical curve because the leakage current is very large. There is a problem that accuracy in detection of the temperature of the primary semiconductor element is low for these reasons. In addition, there are a problem that the electrostatic tolerance of the temperature detecting diode is low and a problem that the response speed of the primary semiconductor element to temperature change is slow, because the temperature detecting diode formed on the insulating film is small in size. In addition, there is a problem that the number of production steps increases remarkably. Particularly when the primary semiconductor element is a trench gate type element, it is impossible to form the temperature detecting diode from doped polysilicon though the doped polysilicon is generally used for the gate electrode. That is, there is a problem that the number of production steps further increases because it is necessary to form the temperature detecting diode by laminating polysilicon separately from the gate electrode.